The present invention relates to an apparatus for diagnosing failure of a sensor that detects the temperature of cooling water of an internal combustion engine.
A radiator mounted on a vehicle supplies cooling water to an internal combustion engine (hereinafter referred to as “engine”) to cool the engine. A sensor for detecting temperature of the cooling water is provided on the engine. This sensor will be hereinafter referred to as an “engine water temperature sensor.” The temperature detected by the engine water temperature sensor is used for various controls of the engine. Appropriate controls of the engine require accurate detection of the engine water temperature.
Japanese Patent Application Unexamined Publication (Kokai) No. 2000-45851 shows a method for detecting a failure of the engine water temperature sensor. According to the method, an output of the engine water temperature sensor is monitored over a predetermined time period. If the output of the sensor changes beyond a predetermined range, it is determined that the sensor is normal. If changes in the output of the sensor are within the predetermined range, it is determined that the sensor is faulty.
According to a method shown in Japanese Patent Application Unexamined Publication (Kokai) No. 2000-282930, an elapsed time after the engine stops is measured. If an output of the engine water temperature sensor is greater than a predetermined value when a predetermined time has elapsed, it is determined that the engine water temperature sensor is stuck at a higher temperature level. When the engine starts before the predetermined time elapses, an output of the engine water temperature is obtained. If the obtained output is lower than a predetermined value, it is determined that the engine water temperature sensor is stuck at a lower temperature level.
When an elapsed time from the time when the engine stops in the previous operating cycle is short and operating conditions of the engine are similar to the previous operating cycle, an output of the engine water temperature sensor may change little. In such a case, the conventional methods may incorrectly determine a normal engine water temperature sensor as being faulty.
The conventional methods require a timer for measuring an elapsed time from the time when the engine stops. Providing such a timer leads to an additional cost. Since detection of a failure of the engine water temperature sensor requires that a predetermined time elapses after the engine stops, the frequency of performing the failure detection process is limited.